The MTX H&K VP9 / P30 21 Round Magazine Extender is compatible with factory magazines and installs in seconds. This XTech extender converts HK 15 or 17 round magazines to 21 rounds. Unlike other extenders on the market, the Patented design of the MTX permits for a consistent internal wall throughout the extender, which is smaller than the magazine tube. The included spring was engineered specifically for the VP9 / P30 to allow for unmatched reliability and last round bolt hold open. XTech Tactical's MTX line of extenders are simply unmatched for reliability, ease of installation, and durability! Mfg: Xtech Tactical